1. Field of the Invention
The present invention relates to an apparatus and method for processing optical signals, and more particularly to an optical signal processing apparatus used as a microwave filter, a TM-TE mode converter, an optical beam scanner, or an electromagnetic field sensor such as a surge current sensor for detecting high-speed surge, as well as to an optical signal processing method used in such an optical signal processing apparatus.
2. Background Art
A conventional optical signal processsing apparatus of one type is disclosed in Tsutsumi, et al., IEICE Transactions, J76-C-1, 114, 1993. FIG. 10 is a schematic diagram of the disclosed optical signal processing apparatus. The optical signal processing apparatus 1 shown in FIG. 10 comprises a YIG (yttrium iron garnet) thin film 2, which serves as a magnetic body, formed on one main surface of a GGG substrate 3. A laser beam generated by a semiconductor laser 4 and having a wavelength of 1.3 xcexcm is input to one side surface of the YIG thin film 2 via a polarizer 5. The input laser beam is output from the opposite side surface of the YIG thin film 2. The output laser beam is received by a Ge photodiode 7 via a analyzer 6 and is then detected by a lock-in amplifier 8. Further, a linear antenna 9 is provided as a transducer on the main face of the YIG thin film 2. Microwaves generated by a microwave oscillator 10 are fed to the antenna 9 via a PIN diode 11 and a GaAs microwave monolithic-IC amplifier 12. Thus, a high frequency magnetic field is excited in the YIG thin film 2. In the optical signal processing apparatus 1, in order to facilitate receipt of signals and increase the SN ratio, the microwave generated at the microwave generator 10 is subjected to amplitude modulation at a frequency of, for example, 1000 Hz, performed by the PIN diode 11 and a low frequency oscillator 13. Therefore, the high frequency magnetic field generated by the microwave induces the magneto-optical effect in the YIG thin film 2 or the like, which in turn induces Faraday rotation, and thus the modulated laser signal is detected.
A conventional optical signal processing apparatus of another type is disclosed in C. S. Tsai, et al., Appl. Phys. Lett. 47, 651, 1985. FIG. 11 is a schematic diagram of the disclosed optical signal processing apparatus. In contrast with the optical signal processing apparatus shown FIG. 10, in the optical signal processing apparatus shown FIG. 11, two antennas 9 and 9xe2x80x2 each formed of a strip line are provided on the main face of the YIG thin film 2 such that they are separated from each other. One antenna 9 is used for generation of microwaves, and the other antenna 9xe2x80x2 is used for detection. In the optical signal processing apparatus shown FIG. 11, a TM-mode optical signal input to the YIG thin film 2 is converted into a TE-mode optical signal by the magneto-optical effect in the YIG thin film 2 or the like.
In each of the above-described optical signal processing apparatuses, a transducer in the form of the antenna 9 is used as means for applying microwaves onto the YIG thin film 2 serving as a magnetic body. Thus, microwaves are generated in a direction perpendicular to the antenna 9, and magnetostatic waves (MSW) are excited within the YIG thin film 2 due to the microwaves. Thus, optical modulation or TM-TE mode conversion is performed.
However, in the prior art techniques, since the efficiency in modulating an optical signal by microwaves is low, a high SN ratio cannot be obtained.
Further, in the prior art techniques, the propagation characteristics of microwaves cannot be changed freely. Therefore, manufacturing a filter device having desired filter characteristics has been difficult.
Moreover, according to the prior art techniques, it has been difficult to manufacture a simple electromagnetic field sensor that can cope with high-speed surge current such as current induced by lightning.
In view of the foregoing, an object of the present invention is to provide an optical signal processing apparatus and method which can improve the SN ratio and the efficiency in modulating an optical signal by microwaves.
In order to achieve the above object, the present invention provides an optical signal processing apparatus in which microwaves are applied to a strip line formed on a magnetic body in order to process an optical signal propagating through the magnetic body.
In the optical signal processing apparatus of the present invention, a magnetic field may be applied to the magnetic body. In this case, the general propagation direction of a light signal within the magnetic body coincides with the application direction of the magnetic field, and the propagation direction of microwaves is substantially perpendicular thereto. Alternatively, the general propagation direction of a light signal within the magnetic body, the application direction of the magnetic field, and the propagation direction of microwaves are perpendicular to one another.
An optical detector may be added to the optical signal processing apparatus of the present invention in order to form a filter. In this case, the optical detector may be selected from the group consisting of a photodiode, a phototransistor, a photoelectric tube, and a photo multiplier.
A TM-TE mode converter may be formed from the optical signal processing apparatus of the present invention.
Also, an optical beam scanner may be formed from the optical signal processing apparatus of the present invention.
Further, an electromagnetic field sensor may be formed from of the optical signal processing apparatus of the present invention.
The present invention also provides an optical signal processing method in which microwaves are applied to a strip line formed on a magnetic body in order to process an optical signal propagating through the magnetic body.
In the optical signal processing apparatus and method of the present invention, ferrite is used for the magnetic body. For example, ferrite having an iron-garnet structure (represented by M3Fe5O12, where M is a metal or metalloid) such as YIG (Y3Fe5O12) may be used.
In the optical signal processing apparatus and method of the present invention, a bulk monocrystal or a monocrystalline thin film may be used as the magnetic body.
According to the present invention, the following advantageous effects are attained.
1) The efficiency in modulating an optical signal by microwaves is high, so that a high SN ratio is obtained.
2) Since a high Q value is attained, a narrow band filter characteristic is obtained.
3) A TM-mode optical signal can be efficiently converted into a TE-mode optical signal.
4) There can be performed scanning in which the propagation direction of a light beam is changed.
5) Surge current such as current induced by lightning can be detected at high speed.